The present invention concerns an electric heating element of the enclosed type and more particularly to the integration of a heat safety device into this element.
In the field of heating elements of the enclosed type, i.e. having a tubular envelope at the interior of which is placed a heating element embedded in an insulator, there is known, from the document DE 34 33 688, a tubular heating body containing a spiral resistive heating wire enclosed in a powder of insulating material such as magnesia. A heat protection element is connected in series with the heating spiral at the interior of the tube. This protection element is a fusible bead that, in the case of overheating, frees a compression spring permitting definitive interruption of the circuit for supplying the heating spiral.
This heating element thus corresponds to a fuse placed in series with the heating element in the tubular envelope, requiring an intermediate connection. Furthermore, confinement of the fuse within the heating element is a source of possible re-ionization of gas around the fuse when this latter is vaporized by overheating of the device, being able to lead, within the insulating material, to a reclosing of the electric supply circuit that has just been opened. This possibility of dysfunctioning of the safety device must be avoided as much as possible because its consequences can be serious.
The document JP 6267641 provides an element of response to the problems mentioned above. The heating element presented in this document is shown in FIG. 1. Thus, a heating element 70 has a spiral resistance 72 contained in a first tube 71 within an insulating powder 73. The spiral resistance 72 is held by two connection pins 78a and 78b through two sealing beads 74a and 74b arranged in the extremities of this first tube 71. A heat protection fuse 77 is housed between bead 74b and a third end bead 76b arranged in a second tube 75 containing the first tube. This fuse is connected to pin 78b as well as a pin 78c connected to an electric supply circuit. At its opposite extremity, a fourth bead 76a, traversed by the pin 78a assures sealing of the second tube.
In this device, the fuse is certainly not housed within the heating element, but is nonetheless confined between two sealing beads, which does not eliminate completely the risks of re-ionization of the surrounding gas after triggering, bead 76b constituting particularly a thermal brake. In addition, the insertion of the fuse requires a specific connection. Overall, four beads and two tubes are thus necessary to obtain a heating element having a heat fuse at one of its extremities, which increases in a notable manner the selling price of the assembly.
The object of the present invention is to overcome the drawbacks of the prior art by presenting an electric heating device constituted by a tubular envelope, at the interior of which is housed a heating element enclosed by an insulator, said heating element being connected to an electric supply circuit by connection rods and pins, said tubular envelope being provided with two cylindrical sealing beads arranged at its extremities, at least one of these beads, called a specific bead, being in direct contact with the insulator enclosing the heating element, characterized in that the specific bead is axially traversed by an opening into which is inserted a heat fuse connected electrically in series with the heating element.
By this arrangement, the number of constituent elements is reduced since one bead assures at the same time a tight seal of the insulator enclosing the heating element, but also maintenance and protection of the housing of the fuse. Thus, without a supplemental element other than the tubular envelope and the two sealing beads, it is permitted to constitute, not only the heating assembly, but equally the addition of a heat fuse. This arrangement avoids, moreover, wiring between the heating element and the fuse within the device, just like the specific means for maintenance and protection of the fuse in the product.
Advantageously, the heating element is in spiral form and mounted under tension in the tubular envelope.
This configuration permits notably the sealing beads, connected to the ends of the heating element, to be maintained by being supported on the extremities of the tubular envelope without a specific part.
Advantageously, the opening has a narrowing at the level of the extremity connected with the interior of the tube, against which the fuse is positioned.
Together with the traction effect exerted by the spiral element, the narrowing formed in the opening permits correct and systematic positioning of the fuse in the opening, without any particular mechanical part. This equally permits assurance of the sealing of the tube with respect to the insulator enclosing the heating element.
According to a preferred embodiment of the invention, the heat fuse has a housing within which is housed a compression spring exerting a pressure between, on the one hand, a heat block situated at the outer extremity of the housing and in contact with an external connection pin and on the other hand a first central sliding disc pushed while resting against an internal connection rod, and this against a restoring force exerted by an opening spring coaxial with the connecting rod.
This association permits, when the heat block is vaporized, opening of the electric supply circuit of the heating element, with the aid of the two springs, compression and opening, releasing while carrying away the first contact disc, into the space left free by the block, thus out of contact with the connecting rod.
Advantageously, the connecting rod is surrounded, within the heat fuse, by an insulator, equally permitting centering and/or maintaining the connecting rod at a distance from the housing of the fuse.
Advantageously, the heat block is situated at the extremity of the fuse housing situated outside of the tubular envelope.
This permits a better controlled reactivity of the protection constituted by the heat fuse, by slightly spacing it from the heating element.
Moreover, direct contact between the block and one of the external connection pins avoids re-ionization of the gas produced during melting of the block, notably by permitting a rapid flow of the heat generated.
In addition, this thermal escape, moreover relative, equally guarantees a better aging of the fuse, preventing its untimely triggering at a temperature lower than that for which it has been calibrated.